Lithium secondary batteries are excellent in terms of energy density, output density, etc. and are effective for a size reduction and a weight reduction. There is hence a rapidly growing demand for the use of lithium secondary batteries as the power sources of portable appliances such as notebook type personal computers, portable telephones, and handy video cameras. Lithium secondary batteries are attracting attention also as power sources for electric vehicles or for leveling the load of electric power, etc. In recent years, there is a rapidly growing demand for the use of the batteries as power sources for hybrid electric vehicles. Especially for use in electric-vehicle applications, the batteries are required to be excellent in terms of low cost, safety, life (in particular, high-temperature life), and load characteristics, and improvements in material are desired.
A substance having the function of being capable of elimination and insertion of lithium ions is usable as a positive-electrode active material among the materials which constitute a lithium secondary battery. There are various kinds of positive-electrode active materials, and these active materials each have features. Common subjects for performance improvements include an improvement in load characteristics, and there is a strong desire for improvements in material.
Furthermore, there is a need for a material which is excellent in terms of low cost, safety, and life (in particular, high-temperature life) and which has a satisfactory balance among performances.
At present, lithium-manganese composite oxides having a spinel structure, lamellar lithium-nickel composite oxides, lamellar lithium-cobalt composite oxides, and the like have been put to practical use as positive-electrode active materials for lithium secondary batteries. The lithium secondary batteries employing these lithium-containing composite oxides each have both advantages and disadvantages concerning battery characteristics. Specifically, the lithium-manganese composite oxides having a spinel structure are inexpensive and relatively easy to synthesize and give batteries having excellent safety, but these batteries have a low capacity and are inferior in high-temperature characteristics (cycle characteristics, storability). The lamellar lithium-nickel composite oxides attain a high capacity and excellent high-temperature characteristics, but have drawbacks, for example, that these composite oxides are difficult to synthesize and give batteries which have poor safety to require care when stored. The lamellar lithium-cobalt composite oxides are easy to synthesize and attain an excellent balance among battery performances and, hence, batteries employing these composite oxides are in extensive use as power sources for portable appliances. However, insufficient safety and a high cost are serious drawbacks of the lamellar lithium-cobalt composite oxides.
Under such current circumstances, a lithium-nickel-manganese-cobalt composite oxide having a lamellar structure has been proposed as a promising active material in which the drawbacks of those positive-electrode active materials have been overcome or minimized and which attains an excellent balance among battery performances. Especially under the recent situation in which a cost reduction, an increase in voltage, and higher safety are increasingly required, the proposed composite oxide is regarded as a promising positive-electrode active material which is capable of satisfying all the requirements.
Hitherto, attempts have been made to improve the properties of a lithium-nickel-manganese-cobalt composite oxide as a positive-electrode active material by adding a compound which contains sulfur element to the composite oxide (see patent documents 1 to 5).
Patent document 1 discloses the following. With respect to LixMyO2 synthesized after basic cobalt is obtained by reacting an aqueous cobalt sulfate solution with an aqueous sodium hydrogen carbonate solution, taking out the resultant precipitate by filtration, and water-washing and drying the precipitate, use of the LixMyO2 which contains sulfuric acid radicals (SO4) from a starting material in a specific amount as a positive-electrode active material is effective in preventing the aluminum foil used as a current collector from corroding and in improving battery performances.
Patent document 2 discloses that self-discharge characteristics and storability can be improved by mixing LiNiaCobMcO2 with AlX(SO4)2.12H2O and heat-treating the mixture to thereby coat the positive-electrode active material with AlX(SO4)2.
Patent document 3 discloses that safety, discharge capacity, and cycle characteristics can be improved by coating a lithium-transition metal composite oxide having a spinel manganese structure with sulfur by dispersing the lithium-transition metal composite oxide in water, adding a metallic ingredient and sulfur to the dispersion while controlling the pH to form a coating layer through a precipitation reaction, subsequently taking out the particles by filtration, and then drying the particles.
Patent document 4 discloses a technique in which transition metal sources for a lithium-transition metal composite oxide of the LiNiMnCoO2 type are mixed with a sulfur-containing compound and the mixture is burned after addition of a lithium source thereto, thereby producing a lithium-transition metal composite oxide powder having a lowered pH.
Patent document 5 discloses that gas evolution and an increase in internal resistance which occur during high-temperature storage can be inhibited or reduced by mixing a lithium-transition metal composite oxide of the LiCoO2 type with a compound that has a phosphorus or sulfur atom and heat-treating the mixture at 900° C.